With this type of engine it has been usual that the intake valve is kept more or less in its open condition, in the course of the compression stroke, with the piston moving upwards, in succession to the suction stroke wherein the piston moves downwards, until the piston reaches near its upper dead point, at a crank angle of approximately 360.degree..
In other words, the full closing time of the intake valve can be slightly delayed towards the upper dead point, in comparison with that of an ordinary or compression-type engine.
Thus the decompression-type, four-cycle engine operates so that, at the time of starting, with the piston moving upwards in the compression stroke, the compressed mixture above the piston is vented therethrough, so that no excessive compression pressure is produced within the combustion chamber. Consequently, the starting operation by means of the kick pedal or the like can be facilitated as to the encountered load, and starting becomes easy and smooth.
With this arrangement, however, when the engine is then transferred to its own operation, large compression pressures cannot be obtained, in almost the same manner as with starting, so that an inferior output characteristic cannot be avoided.
When the intake valve is delayed in its closing time, the resultant compression pressure is much lower than that obtained with an ordinary engine, and thus the output characteristic is lower.